How the conversion works
Food energy comes in two units, and they convert with one exact factor. A kilocalorie (kcal), the unit everyone calls a calorie, is defined as exactly 4.184 kilojoules. So multiplying kcal by 4.184 gives kJ, and dividing kJ by 4.184 gives kcal. There is nothing approximate about it: the calorie was fixed by definition at 4.184 joules, so the conversion is as exact as inches to centimetres.
Type into either box above and the other updates instantly. A rough mental version: kilojoules are a bit more than 4 times the kilocalories, so knock the last digit off a kJ figure and multiply what is left by 2.4 to land close to the kcal value.
Worked examples
Reading the numbers on a UK label
UK food labels, following rules carried over from EU regulations, declare energy in both units with kilojoules first: something like "Energy 1,590 kJ / 380 kcal per 100 g", usually with a per-portion column alongside. The percentage next to it compares the portion against the reference intake of 8,400 kJ (2,000 kcal), which is a general benchmark used for labelling rather than a personal target.
Different countries chose different sides of the same coin. Australia and New Zealand label in kilojoules, so a visitor reading 1,800 kJ on a cereal box is looking at about 430 kcal. The United States labels only in Calories, capital C, which are kilocalories by another name. If you compare products across borders, or follow a recipe from an Australian site, this converter translates between all three labelling styles.
One thing to watch when comparing labels is the basis, not the unit: per 100 g figures compare products fairly, while per-portion figures depend on how generous the manufacturer's idea of a portion is. Convert the units first, then make sure both numbers describe the same amount of food.
Calories, kilocalories and joules
The naming muddle is historical. The original calorie was defined as the energy needed to warm one gram of water by one degree Celsius, a tiny amount, and food energy was measured in thousands of them, kilocalories. Everyday speech dropped the "kilo", so the calorie on a menu is really a kilocalorie, and scientific writing sometimes marks the food version as Calorie with a capital C. The joule is the SI unit of energy, named after the Manchester physicist James Joule, and it connects food to every other kind of energy: the kilowatt hour on your electricity bill is 3,600 kJ, which is about 860 kcal. Labels write kcal to keep things unambiguous, and this page sticks to kcal and kJ for the same reason. If you are converting recipe measurements as well as energy, our cups to grams converter covers baking amounts, and the weight converter handles grams, ounces and pounds.
Frequently asked questions
How many kilojoules are in a calorie?
One kilocalorie (the food calorie) is exactly 4.184 kilojoules. So a 250 kcal snack is 1,046 kJ. To go the other way, divide by 4.184: 1,000 kJ is about 239 kcal.
Are calories and kilocalories the same thing?
On food labels, yes. The calorie people talk about in food is technically a kilocalorie, which is 1,000 of the small calories used in chemistry. Labels write it as kcal to be precise, but calorie, Calorie and kcal all mean the same unit in everyday food contexts.
Why do UK food labels show both kJ and kcal?
UK labelling rules, carried over from EU regulations, require energy to be declared in kilojoules and kilocalories, in that order, per 100 g or 100 ml and usually per portion too. The kilojoule is the international scientific unit of energy, while the kilocalorie is the one most people actually recognise.
What is 2,000 kcal in kilojoules?
2,000 kcal multiplied by 4.184 is 8,368 kJ. Labels round the reference intake to 8,400 kJ, which is why you see the pair 8,400 kJ / 2,000 kcal printed on UK and EU packaging.
Which countries use kilojoules instead of calories?
Australia and New Zealand label food energy in kilojoules, with kilocalories optional, so an Australian label might say 1,800 kJ where a UK label would lead with 430 kcal. The United States goes the other way and shows only Calories. This converter bridges all three styles.